1. Field of the Invention
This application relates generally to optical coherence tomography (OCT) imaging methods and apparatus and, more specifically, to an OCT imaging method and apparatus utilizing dynamic focus and/or windowed averaging.
2. Description of Related Art
OCT is an imaging technique capable of acquiring sub-surface images of a subject at micrometer resolutions. In ophthalmological applications, OCT is utilized to generate cross-sectional images of portions of an eye, including the posterior (e.g. retina) and/or anterior (e.g., cornea, crystalline lens, etc.) regions. While clinical applications have traditionally focused on the posterior region, there is growing interest in OCT imaging of the vitreous, the choroid, the sclera, the crystalline lens, and, essentially, all portions of the eye. For instance, the vitreous, and particularly the condition of the vitreous, can contribute to various blinding eye conditions such as retinal detachment, macular holes, and diabetic retinopathy. However, OCT signal intensity of the vitreous, the crystalline lens, and, often, the choroid and/or sclera is weak, i.e., close to a background noise level.
In clinical practice, commonly used OCT instruments utilize spectral domain technology, and thus have a high roll-off in sensitivity with depth. Accordingly, imaging wider expanses of the vitreous becomes difficult. With the emergence of swept source OCT, which has a lower roll-off in sensitivity with depth, imaging the vitreous is potentially easier. However, the illumination beam is constrained by the physics of how light is focused. For instance, a high numerical aperture system focuses light to a relatively smaller spot, but results in a higher cone angle for the beam of light. The higher cone angle in turn reduces a range over which the beam of light is at an acceptable diameter (i.e. provides suitable resolution). A lower numerical aperture system has a longer zone in which the beam is narrow, but the minimum diameter of the focused light beam is comparatively larger. Thus, lower numerical aperture systems typically have reduced transverse or lateral resolution than high numerical aperture systems.
Moreover, the dimensions of the beam of light lead to other effects. In the vitreous, for example, the reflective structures include collagen fibers and individual cells, each of which is very small. A larger illumination beam leads to reflections from these reflective, yet small, structures, but the same beam also illuminates surrounding areas that are potentially non-reflective. As a consequence, the total light reflected back to the OCT instrument is a small proportion of the light entering the eye. The total light used in diagnostic instruments is severely limited by the need to maintain safety limits and, therefore, cannot be increased to compensate for the inefficiencies of the illumination system.